Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a transmission line probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. In the case of the probe being a flexible single conductor probe, the probe may be kept substantially vertical by means of a weight attached to the bottom end of the flexible single conductor probe.
An electromagnetic transmit signal is generated by a transceiver and propagated by the probe towards the surface of the product in the tank, and an electromagnetic reflection signal resulting from reflection of the transmit signal at the surface is propagated back towards to the transceiver.
Based on the transmit signal and the reflection signal, the distance to the surface of the product can be determined.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the variation of the phase difference between a transmitted swept (frequency-modulated) signal and its reflection at the surface. The latter type of system is generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type. The pulsed systems commonly used for GWR use a train of short DC pulses (˜1 ns) processed in a way called TDR (Time Domain Reflectometry).
In any case, the transmit signal is typically not only reflected at the impedance transition constituted by the interface between the tank atmosphere and the surface of the product, but at several other impedance transitions encountered by the transmit signal. In the case of a GWR-system, one such impedance transition typically occurs at the connection between the transceiver and the probe and another such impedance transition occurs at the bottom end of the probe. The impedance transition at the bottom end of the probe may make it difficult to accurately determine filling levels close to the bottom end of the probe.